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This article was reported by Julie Bosman, Larry Rohter and Katharine Q. Seelye and was written by Ms. Seelye.

By mid-March, Senator Hillary Rodham Clinton’s campaign knew it had a problem with what it had once assumed was a reliable firewall — its support among superdelegates.

The fight for pledged delegates for the Democratic nomination was essentially over. Senator Barack Obama was ahead, after winning a series of caucuses in states that Mrs. Clinton virtually ignored.

Still, it became apparent that neither he nor Mrs. Clinton could claim the presidential nomination with pledged delegates alone, and the two would need superdelegates — elected officials and party activists — to fill the gap.

For Mrs. Clinton in particular, that signaled danger. The commanding lead she had held in superdelegates at the start of the contests — she was about 100 ahead of Mr. Obama — had dwindled by mid-March, to 12.

And superdelegates were showing an independence that the Clinton campaign had not counted on, not quite buying her argument that she was more electable than Mr. Obama.

The break in Mrs. Clinton’s supposed firewall turned out to be one of the most important factors in her campaign.

“Sure, Senator Clinton was the favorite early on, but that was simply because of the institutional support that she already had,” said Jason Rae of Wisconsin, a superdelegate who endorsed Mr. Obama in February. “In the beginning, people were unsure of Senator Obama. But as they continued to see primary after primary, and him excelling, and him attracting all these new voters, I think the superdelegates really started feeling more comfortable with him.”

Of all the assumptions the Clinton campaign made going into the race, its support among the party establishment seemed like a safe bet. Many of the superdelegates, who help pick the nominee at the convention in August, came of age during the Bill Clinton presidency. Many were personal Clinton loyalists, cultivated to help deliver the vote.

But the Obama campaign convinced many superdelegates that they should follow the voters’ will in making their endorsements. To the puzzlement and increasing frustration of the Clinton camp, few flowed her way. Her campaign never recovered from its string of losses through February. By the time she started winning again, with Ohio on March 4, her support among superdelegates hardly inched up.

At the same time, Mr. Obama posted a small but steady increase, culminating in a flood that surged on Tuesday and helped him claim the nomination.

In retrospect, relying on superdelegates as a firewall was flawed, said superdelegates who endorsed Mr. Obama.

Representative David E. Price, a superdelegate from North Carolina, said the idea that Mrs. Clinton could amass enough superdelegates to overturn the verdict of pledged delegates “was never in the cards.”

Don Fowler, a former party chairman and a superdelegate who had supported Mrs. Clinton, said as much in a memo to the campaign on March 11 predicting that at the end of the primaries Mr. Obama would have about 100 more pledged delegates than Mrs. Clinton.

Mr. Fowler said that “everything humanly possible should be done” to keep that number below 100, because it would be easier to persuade superdelegates that the two were essentially tied.

The Clintons certainly tried, interviews with two dozen superdelegates found. Many said that the Clintons had intensely pressured them and that their endorsements became a test of personal loyalty, subject to a hard sell. At the same time, many said they were drawn to the Obama campaign’s excitement.

So even during the Obama campaign’s darkest days — an eight-week stretch between Ohio and the Pennsylvania primary on April 22, during which Mr. Obama had called rural voters “bitter” and had to renounce his ties to his former pastor because of racial comments — more superdelegates were lining up with him than with her.

The Obama campaign skillfully managed the flow. Richard Machacek, a farmer and superdelegate from Iowa, for instance, said he told the Obama campaign on a Monday, April 29, that he was endorsing Mr. Obama. The campaign waited until Tuesday afternoon, the same day that Mr. Obama held a news conference to angrily renounce Reverend Wright, to announce Mr. Machacek’s endorsement.

“I don’t know if that was on my mind,” Mr. Machacek said of the timing. “But he needed it more then than he did before.”

David Wilhelm, Mr. Clinton’s first chairman of the Democratic Party, endorsed Mr. Obama in mid-February because, he said, he recognized the race might come down to them and he wanted to send a message to other superdelegates that it was time to support Mr. Obama.

Representative Ben Chandler of Kentucky, who came out for Mr. Obama on April 29, said he timed his endorsement to an “unusually critical” moment.

Mr. Chandler made a reference to the controversial Rev. Jeremiah A. Wright Jr. “Reverend Wright’s second incarnation,” Mr. Chandler said. “I did step forward then, because I thought it would be particularly important to him at that time. They seemed to be very happy about it.”

It was the sense among many superdelegates that they should follow the voters’ lead rather than loyalty to the Clintons that prompted many to come out on Mr. Obama’s behalf.

Patsy Arceneaux, a National Committee member from Louisiana who had a friendship with the Clintons, was persuaded early this year to support Mrs. Clinton. But when Mr. Clinton made what she saw as racially inflammatory comments in South Carolina, Ms. Arceneaux said she developed serious misgivings about supporting Mrs. Clinton.

After switching to Mr. Obama two weeks ago, the Clinton campaign bombarded her with dozens of calls, she said. “You can’t imagine how stressful this has been,” Ms. Arceneaux said. “It had gotten to where my life had just been taken over by this.”

Debbie Marquez, a superdelegate from Colorado, said she had made up her mind to shift to Mr. Obama, largely because he opposed the Iraq war from the start. The ex-president called and talked for 45 minutes, she said.

“When people talk about the finger wagging and lecturing in his speeches, I kind of felt that was going on over the phone,” Ms. Marquez said.